UV light emitting diode and method of fabricating the same

ABSTRACT

Exemplary embodiments provide a UV light emitting diode and a method of fabricating the same. The method of fabricating a UV light emitting diode includes growing a first n-type semiconductor layer including AlGaN, wherein growth of the first n-type semiconductor layer includes changing a growth pressure within a growth chamber and changing a flow rate of an n-type dopant source introduced into the growth chamber. A pressure change during growth of the first n-type semiconductor layer includes at least one cycle of a pressure increasing period and a pressure decreasing period over time, and change in flow rate of the n-type dopant source includes increasing the flow rate of the n-type dopant source in the form of at least one pulse. The UV light emitting diode fabricated by the method has excellent crystallinity.

PRIORITY CLAIMS AND CROSS-REFERENCE TO RELATED APPLICATION

This patent document is a divisional of, and claims priority andbenefits of, U.S. patent application Ser. No. 14/810,464, filed on Jul.27, 2015 now U.S. Pat. No. 9,496,455, and claims priority and benefitsof Korean Patent Application No. 10-2014-0094958, filed on Jul. 25,2014, the contents of each application are incorporated by referenceherein in their entirety.

TECHNICAL FIELD

The disclosure of this patent document relates to a UV light emittingdiode and a method of fabricating the same. Some implementations of thedisclosed technology permit separation of a growth substrate infabrication of the UV light emitting diode and include semiconductorlayers having excellent crystallinity.

BACKGROUND

Light emitting diodes refer to inorganic semiconductor devices that emitlight generated by recombination of electrons and holes. Particularly,ultraviolet (UV) light emitting diodes have been increasingly used in avariety of fields such as UV curing, sterilization, white light sources,medicines, equipment parts, and the like.

UV light emitting diodes emit light having a relatively short peakwavelength (generally, light having a peak wavelength of 400 nm orless). In fabrication of such UV light emitting diodes, an active layeris formed of a material having band gap energy corresponding to arelatively short peak wavelength in order to emit light of therelatively short peak wavelength. For example, the active layer can beformed of AlGaN containing 10% or more of Al as a nitride semiconductor.In addition, if the band gap energy of n-type and p-type nitridesemiconductor layers is lower than energy of UV light emitted from theactive layer, the UV light emitted from the active layer can be absorbedinto the n-type and p-type nitride semiconductor layers in the lightemitting diode. Accordingly, not only the active layer of the UV lightemitting diode but also other semiconductor layers disposed in a lightemitting direction of the light emitting diode are formed to have an Alcontent of 10% or more.

SUMMARY

Exemplary embodiments provide a UV light emitting diode that includessemiconductor layers having excellent crystallinity, and a method offabricating the same.

In one aspect, a method of fabricating a UV light emitting diode isprovided to include: forming an n-type semiconductor layer on a growthsubstrate within a growth chamber; and forming an active layer and ap-type semiconductor layer on the n-type semiconductor layer, whereinthe forming the n-type semiconductor layer includes growing a firstn-type semiconductor layer including AlGaN, the growing the first n-typesemiconductor layer includes changing a growth pressure within thegrowth chamber and changing a flow rate of an n-type dopant sourceintroduced into the growth chamber, the changing the growth pressureduring growth of the first n-type semiconductor layer includesperforming at least one cycle of a pressure increasing period and apressure decreasing period, and the changing the flow rate of the n-typedopant source includes increasing the flow rate of the n-type dopantsource in a pulse form.

Accordingly, a method of fabricating a UV light emitting diode havingexcellent crystallinity and improved luminous efficacy can be provided.

In some implementations, the growing the first n-type semiconductorlayer can include growing an Al_(x)Ga_((1-x))N layer (0<x<1) in thepressure increasing period and growing an Al_(y)Ga_((1-y))N layer(0<y<1) in the pressure decreasing period, and x gradually decreases ina direction away from the growth substrate, and y gradually increases inthe direction away from the growth substrate.

In some implementations, the changing the growth pressure during growthof the first n-type semiconductor layer includes at least two cycles ofthe pressure increasing period and the pressure decreasing period.

In some implementations, the changing the flow rate of the n-type dopantsource can include supplying the n-type dopant source at a first flowrate during the pressure decreasing period and supplying the n-typedopant source at a second flow rate higher than the first flow rateduring the pressure increasing period, and wherein the changing the flowrate of the n-type dopant source includes supplying the n-type dopantsource in a pulse form.

In some implementations, the changing the flow rate of the n-type dopantsource includes supplying the n-type dopant source in a square waveform.

In some implementations, the changing the growth pressure during growthof the first n-type semiconductor layer forms a triangular waveform or aharmonic waveform.

In some implementations, the forming the n-type semiconductor layerfurther includes maintaining flow rates and growth temperature of Al, Gaand N sources within the growth chamber as constant.

In some implementations, the fabrication method can further include,before the forming the n-type semiconductor layer: forming a GaN layer;forming an AlN layer on the GaN layer, wherein the forming the AlN layercan include supplying an Al source and the N source at constant flowrates into the growth chamber and changing pressure of the growthchamber from a first pressure to a second pressure and vice versa. Here,the first pressure can be different from the second pressure.

In some implementations, the fabrication method can further includeforming an undoped nitride layer before forming the n-type semiconductorlayer.

In some implementations, the forming the undoped nitride layer caninclude alternately stacking an Al_(w)Ga_((1-w))N layer (0<w<1) grown ata first pressure and an Al_(z)Ga_((1-z))N layer (0<z<1) grown at asecond pressure, wherein the first pressure can be different from thesecond pressure.

In some implementations, the forming the undoped nitride layer caninclude changing a growth pressure within the growth chamber over time,and the changing of the growth pressure during growth of the undopednitride layer can include performing at least one cycle including apressure increasing period and a pressure decreasing period.

In some implementations, the forming the n-type semiconductor layer canfurther include forming at least one of a second n-type semiconductorlayer and a third n-type nitride semiconductor layer over a lowersurface and an upper surface of the first n-type semiconductor layer,respectively.

In some implementations, the forming the second n-type semiconductorlayer, the third n-type semiconductor layer, or both includes changingthe flow rate of the n-type dopant source introduced into the growthchamber, and the changing the flow rate of the n-type dopant source caninclude increasing the flow rate of the n-type dopant source in a pulseform.

In some implementations, the changing the growth pressure during thegrowth of the first n-type semiconductor layer can further includemaintaining the growth pressure at a constant level during at least onepressure maintaining period.

In some implementations, the forming the AlN layer grown at the firstpressure can have a different Al mole fraction than the AlN layerincludes forming the AlN layer having varying Al mole fractionsdepending on pressure condition under which the AlN layer is formed.

In another aspect, a UV light emitting diode is provided to include: asupport substrate; a p-type semiconductor layer disposed over thesupport substrate; an active layer disposed over the p-typesemiconductor layer; and an n-type semiconductor layer disposed over theactive layer, wherein the n-type semiconductor layer includes a firstn-type semiconductor layer including at least a portion having band gapenergy continuously changing in a thickness direction of the firstn-type semiconductor layer.

In some implementations, the first n-type semiconductor layer can have astack structure including an Al_(x)Ga_((1-x))N layer (0<x<1) and anAl_(y)Ga_((1-y))N layer (0<y<1), wherein x gradually decreases in adirection away from the growth substrate and y gradually increases inthe direction away from the growth substrate.

In some implementations, the n-type semiconductor layer can furtherinclude a second n-type semiconductor layer disposed over a lowersurface of the first n-type semiconductor layer, a third n-type nitridelayer disposed over an upper surface of the first n-type semiconductorlayer, or both second and third n-type nitride layers disposed over thelower and upper surfaces respectively.

In some implementations, the second n-type semiconductor layer, thethird n-type nitride layer, or each of the second and third n-typesemiconductor layers can have a stack structure including anA_(u)Ga_((1-u))N layer (0<u<1) with a first n-type-doping impurityconcentration and an A_(v)Ga_((1-v))N layer (0<v<1) with a secondn-type-doping impurity concentration, and the first n-type dopingimpurity concentration is higher than the second n-type doping impurityconcentration.

In some implementations, the first n-type semiconductor layer caninclude a region that has constant band gap energy in a thicknessdirection of the first n-type semiconductor.

Embodiments of the disclosure provide a method of fabricating a UV lightemitting diode, which can improve crystallinity of the UV light emittingdiode through a relatively simple and easy process. In addition,embodiments of the disclosure provide a UV light emitting diode, fromwhich a growth substrate is removed, thereby providing excellentluminous efficacy. Further, some implementations of the UV lightemitting diode can prevent damage due to stress within semiconductorlayers, thereby providing improved reliability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 to FIG. 10 are sectional views and diagrams illustrating anexemplary method of fabricating a light emitting diode and an exemplarylight emitting diode fabricated according to some embodiments of thedisclosure.

FIG. 11 and FIG. 12 are sectional views illustrating an exemplary methodof fabricating a light emitting diode and an exemplary light emittingdiode fabricated according to some embodiments of the disclosure.

FIG. 13 and FIG. 14 are sectional views illustrating an exemplary methodof fabricating a light emitting diode and an exemplary light emittingdiode fabricated according to some embodiments of the disclosure.

FIG. 15 to FIG. 22 are sectional views illustrating an exemplary methodof fabricating a light emitting diode and an exemplary light emittingdiode fabricated according to some embodiments of the disclosure.

FIG. 23a to FIG. 24b are diagrams depicting change in band gap energy ina light emitting diode according to some embodiments of the disclosure.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings. The followingembodiments are provided by way of example so as to facilitate theunderstanding of various implementations of the disclosed technology.Accordingly, the present disclosure is not limited to the embodimentsdisclosed herein and can also be implemented in different forms. In thedrawings, widths, lengths, thicknesses, and the like of elements can beexaggerated for clarity and descriptive purposes. When an element orlayer is referred to as being “disposed above” or “disposed on” anotherelement or layer, it can be directly “disposed above” or “disposed on”the other element or layer or intervening elements or layers can bepresent. Throughout the specification, like reference numerals denotelike elements having the same or similar functions.

Composition ratios, growth methods, growth conditions, thicknesses, andthe like disclosed in the following descriptions are provided asexamples, and other implementations are also possible. For example, forAlGaN, various composition ratios of Al and Ga can be used depending onthe need of a person having ordinary knowledge in the art (“thoseskilled in the art”). Furthermore, semiconductor layers disclosedhereinafter can be grown by various methods such as Metal OrganicChemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE), HydrideVapor Phase Epitaxy (HVPE), or the like. In the following exemplaryembodiments, semiconductor layers are grown in the same chamber byMOCVD, and conventional sources with predetermined composition ratioscan be used and introduced into the chamber. For example, a Ga sourcecan include TMGa, or TEGa, and the like; an Al source can include TMA,or TEA, and the like; an In source can include TMI, or TEI, and thelike; and an N source can include NH₃. However, such compositions areprovided as examples only and other implementations are also possible.

In fabrication of UV light emitting diodes, a sapphire substrate is usedas an exemplary growth substrate. When an Al_(x)Ga_((1-x))N layer(0.1≦x≦1) is grown on the sapphire substrate, cracking or breakingoccurs due to thermal or structural deformation resulting from a high Alcontent. This problem results from lattice mismatch and/or a differencein coefficient of thermal expansion between the sapphire substrate andthe Al_(x)Ga_((1-x))N layer (0.1≦x≦1). In the related art, in order tominimize occurrence of such problems, a light emitting diode isfabricated by forming an AlN layer at high temperature on the sapphiresubstrate or forming an AlN/AlGaN superlattice layer on the sapphiresubstrate, followed by forming an n-type semiconductor layer includingan Al_(x)Ga_((1-x))N (0.2≦x≦1), an active layer, and a p-typesemiconductor layer. For example, U.S. Pat. No. 7,192,849 B2 and thelike disclose a structure including an AlInGaN superlattice layer usinga precursor flux pulse.

However, U.S. Pat. No. 7,192,849 B2 and the like disclose onlyadjustment of a flow of an atomic source gas with a pulse in the methodof growing the AlInGaN superlattice layer. When the superlattice layeris grown through adjustment of only the flow of the atomic source gas,other remaining atomic source gases can deteriorate reproducibilitythereof, and make it difficult to form a superlattice layer having anintended composition.

Moreover, for a UV light emitting diode including an AlN layer grown ona sapphire substrate, it is difficult to separate the growth substratefrom the semiconductor layers. Generally, when the sapphire substrate isused as the growth substrate, the growth substrate is separated from thesemiconductor layers using a laser lift-off process. However, excimerlaser beams generally used in the laser lift-off process have a longerwavelength than the band gap energy of AlN or a substantially similarwavelength thereto. For example, a KrF excimer laser beam has awavelength of 248 nm, which passes through an AlN layer, and thus cannotbe used for laser lift-off. In addition, an ArF excimer laser beam has awavelength of 193 nm and can be absorbed into the AlN layer. However,since there is not much difference between the wavelength of the ArFexcimer laser beam and a wavelength (about 200 nm) corresponding to theband gap energy of the AlN layer, some of the ArF excimer laser beam canpass through the AlN layer or the AlN/AlGaN superlattice layer.Moreover, the ArF excimer laser cannot provide enough energy to separatethe growth substrate due to low pulse energy thereof.

Therefore, a typical UV light emitting diode has been provided as alateral type light emitting diode or a flip-chip type light emittingdiode including a growth substrate. The lateral type or flip-chip typelight emitting diode includes a partially removed active layer, exhibitspoor heat dissipation, and has low efficiency.

Moreover, since Al_(x)Ga_((1-x))N (0.1≦x≦1) is difficult to grow withgood crystallinity as compared with GaN, the fabricated light emittingdiode has low internal quantum efficiency. Generally, it is known in theart that an Al_(x)Ga_((1-x))N (0.1≦x≦1) layer grown at a hightemperature of 1200° C. or more by metal organic chemical vapordeposition (MOCVD) has improved crystallinity. However, when operated ata high temperature of 1200° C. or more, a conventional MOCVD apparatuscan suffer from reduction in lifespan, and it is difficult to achievestable growth of Al_(x)Ga_((1-x))N (0.1≦x≦1). Therefore, it is difficultto achieve mass production of UV light emitting diodes having goodcrystallinity and high internal quantum efficiency using theconventional MOCVD apparatus.

FIG. 1 to FIG. 9 are sectional views and diagrams illustrating anexemplary method of fabricating a light emitting diode and an exemplarylight emitting diode fabricated according to some embodiments of thedisclosure.

Referring to FIG. 1, a GaN layer 123 is formed on a growth substrate110. In some implementations, a buffer layer 121 can also be formed onthe growth substrate 110 before formation of the GaN layer 123.

The growth substrate 110 can be any substrate capable of growing nitridesemiconductor layers without limitation, and can include, for example, asapphire substrate, silicon carbide substrate, a spinel substrate, or anitride substrate such as a GaN substrate or an AlN substrate. As anexample, in this embodiment, the growth substrate 110 can be or includea sapphire substrate.

The GaN layer 123 can be grown to a thickness of about 3 μm or less, forexample, about 1 μm, on the growth substrate 110. The GaN layer 123 canbe grown at a temperature of about 900° C. to about 1100° C. and apressure of about 200 Torr within the growth chamber. By forming the GaNlayer 123 on the growth substrate 110, it is possible to employ laserlift-off in a process for separating the growth substrate 110 describedbelow.

Before growth of the GaN layer 123, the buffer layer 121 can be grown toa thickness of about 25 nm or less on the growth substrate 110 at atemperature of about 600° C. and a pressure of 600 Torr. In someimplementations, when the growth substrate 110 is a sapphire substrate,the buffer layer 121 can act as a nucleus layer so as to allow growth ofother semiconductor layers thereon, and relieve stress due to latticemismatch between the sapphire substrate and other semiconductor layersdescribed below. The buffer layer 121 can be omitted, as needed.

Referring to FIG. 2, an n-type semiconductor layer 130 including a firstn-type semiconductor layer 131 is formed.

The first n-type semiconductor layer 131 can be grown on the GaN layer123 within the growth chamber. In addition, the first n-typesemiconductor layer 131 can include an Al_(x)Ga_((1-x))N layer (0<x<1, xbeing not constant) and an Al_(y)Ga_((1-y))N layer (0<y<1, y being notconstant), and can be doped to become an n-type semiconductor layer withn-type impurities such as Si.

Growth of the first n-type semiconductor layer 131 includes introducingan Al source, a GaN source and an N source into the growth chamber, inwhich growth temperature can be set in the range of about 900° C. toabout 1100° C. During growth of the first n-type semiconductor layer131, a flow rate of each of the Al source, the GaN source and the Nsource can be kept constant and the growth temperature can also begenerally kept constant within allowable tolerance.

Furthermore, the growth process of the first n-type semiconductor layer131 can include changing a pressure condition within the growth chamber,and the changing of the pressure within the growth chamber during growthof the first n-type semiconductor layer 131 can include time periodsincluding a pressure increasing period and a pressure decreasing period.During the growth of the first n-type semiconductor layer 131, at leastone cycle of pressure increase and pressure decrease processes can beperformed. Further, the growth process of the first n-type semiconductorlayer 131 can include supplying the n-type dopant source into the growthchamber in the form of multiple pulses.

In this embodiment, the first n-type semiconductor layer 131 can begrown by changing the pressure and the flow rate of the n-type dopantsource without changing other growth conditions.

Next, growth of the first n-type semiconductor layer 131 will bedescribed in more detail with reference to FIGS. 3a to 4 b.

First, referring to FIG. 3a , during growth of the first n-typesemiconductor layer 131, the pressure within the chamber can becontinuously changed, and, as shown therein, the pressure change withinthe chamber can include a cycle of a pressure increasing period 11 inwhich the pressure is increased from a first pressure to a secondpressure and a pressure decreasing period 12 in which the pressure isdecreased from the second pressure to the first pressure. The cycle ofthe pressure increasing period 11 and the pressure decreasing period 12can be repeated at least twice, and the pressure change over time canhave a triangular waveform. The first pressure is lower than the secondpressure. In some implementations, the first pressure can be between 0and 100 Torr, and the second pressure can be between 0 and 300 Torr.

In the pressure increasing period 11, the Al_(x)Ga_((1-x))N layer(0<x<1, x being not constant) can be grown and in the pressuredecreasing period 12, the Al_(y)Ga_((1-y))N layer (0<y<1, y being notconstant) can be grown. For example, as in this embodiment, when thepressure change over time has a triangular waveform in the course ofgrowing the first n-type semiconductor layer 131, the first n-typesemiconductor layer 131 can have a repeated stack structure ofAl_(x)Ga_((1-x))N layers and Al_(y)Ga_((1-y))N layers, and the repeatedstack structure can be or include a superlattice structure.

When only the pressure is changed while keeping other growth conditionsconstant, the composition ratio of the AlGaN layer to be grown is alsochanged. As the pressure increases, the Al content of the AlGaN layer tobe grown is decreased. As such, since the Al content of theAl_(x)Ga_((1-x))N layer (0<x<1, x being not constant) is decreasedduring growth of the Al_(x)Ga_((1-x))N layer in the pressure increasingperiod 11, the Al content of the Al_(x)Ga_((1-x))N layer is graduallydecreased in an upward direction from the growth substrate 110. On thecontrary, since the Al content of the Al_(y)Ga_((1-y))N layer (0<y<1, ybeing not constant) is increased during growth of the Al_(y)Ga_((1-y))Nlayer in the pressure decreasing period 12, the Al content of theAl_(y)Ga_((1-y))N layer is gradually increased in the upward directionfrom the growth substrate 110.

The reason that the Al content of the AlGaN layer is changed dependingupon the pressure within the growth chamber will be describedhereinafter. For example, when TMGa, TMA and NH₃ are used as Al, Ga andN sources, respectively, these sources react to form AlGaN crystalswithin the growth chamber while CH₄ is discharged. However, sometimes,these sources can fail to form the AlGaN crystals during crystal growthof AlGaN and can be discharged outside the growth chamber instead ofreacting in the form of TMGa-NH₃ (Ga(CH₃)—NH₃) or TMA-NH₃ polymer(Al(CH₃)—NH₃). At this time, when the pressure within the growth chamberis relatively decreased, the formation ratio of the polymer is decreasedand the ratio of source molecules contributing to formation of crystalsto source molecules of each of the source gases introduced into thegrowth chamber increases. Since Al exhibits a stronger inclinationtowards CH₃ ligand than Ga, the formation ratio of the polymer becomessensitive to the pressure within the growth chamber. That is, even inthe same state where pressure decreases, the formation ratio and lossratio of the polymer of Al become lower than the formation ratio andloss ratio of the polymer of Ga, thereby increasing an Al mole fractionas compared with Ga during the formation of the AlGaN layer.Accordingly, the Al content of the AlGaN layer increases with decreasingpressure under the same growth conditions excluding the pressure.

When the pressure changes in a triangular waveform as in thisembodiment, the Al content of the Al_(x)Ga_((1-x))N layer and theAl_(y)Ga_((1-y))N layer can change substantially linearly. However, itshould be understood that the present disclosure is not limited theretoand other implementations are also possible. The pressure change canoccur as shown in FIG. 3b, 4a , or 4 b.

As shown in FIG. 3b , the pressure changes like a harmonic waveform. Inthis case, the Al content of the Al_(x)Ga_((1-x))N layer and theAl_(y)Ga_((1-y))N layer can exhibit non-linear variation with thepressure change.

Furthermore, as shown in FIG. 4a , in the pressure change, the pressureincreasing period 21 and the pressure decreasing period 22 can beasymmetrical to each other. For example, duration of the pressureincreasing period 21 can be set to be longer than that of the pressuredecreasing period 22, whereby the Al_(x)Ga_((1-x))N layer can have ahigher thickness than the Al_(y)Ga_((1-y))N layer. Further, the changerate of the Al content in the Al_(x)Ga_((1-x))N layer according to thethickness thereof can be lower than that of the Al content in theAl_(y)Ga_((1-y))N layer according to the thickness thereof.

Furthermore, as shown in FIG. 4b , in the pressure increasing period andthe pressure decreasing period, the pressure can be kept constant for apredetermined period of time. For example, in a first zone 31, thepressure of the growth chamber is increased from a first pressure to asecond pressure over time, and in a second zone 32, the pressure of thegrowth chamber is maintained at the second pressure for a predeterminedperiod of time. Then, in a third zone 33, the pressure of the growthchamber is decreased to the first pressure over time; in a fourth zone34, the pressure of the growth chamber is maintained at the firstpressure for a predetermined period of time; and in a fifth zone 35, thepressure of the growth chamber is decreased to a third pressure overtime. Then, in a sixth zone 36, the pressure of the growth chamber ismaintained at the third pressure for a predetermined period of time; ina seventh zone 37, the pressure of the growth chamber is increased fromthe third pressure to the first pressure over time; and in an eighthzone 38, the pressure of the growth chamber is maintained at the firstpressure for a predetermined period of time. During growth of the firstn-type semiconductor layer 131, the pressure within the growth chamberchanges by including at least one cycle of the pressure increasingperiod and the pressure decreasing period over time. In someimplementations, the pressure within the growth chamber changes tofurther include at least one pressure maintaining period.

Referring back to FIG. 3a , the growth process of the first n-typesemiconductor layer 131 includes changing the flow rate of the n-typedopant source introduced into the growth chamber, and changing the flowrate of the n-type dopant source can include increasing the flow rate ofthe n-type dopant source in the form of at least one pulse. For example,as shown therein, in the pressure increasing period 11, the n-typedopant source can be introduced at a second flow rate into the growthchamber, and in the pressure decreasing period 12, the n-type dopantsource can be introduced at a first flow rate into the growth chamber.The first flow rate can be lower than the second flow rate. When thecycle of the pressure increasing period 11 and the pressure decreasingperiod 12 is repeated in a triangular waveform as in this embodiment,the n-type dopant source can be introduced into the growth chamber in amultiple-waveform corresponding to the pressure change. Each of themultiple-waveform can have a certain period of time, and, for example,as shown in FIG. 3a , the flow rate of the n-type dopant source can beprovided in a square waveform.

The first and second flow rates can be adjusted according to dopingconcentrations of the Al_(x)Ga_((1-x))N layer and the Al_(y)Ga_((1-y))Nlayer, respectively. For example, the first flow rate is set to allowthe Al_(y)Ga_((1-y))N layer to be grown in a substantially undoped stateor in a low doping concentration, and the second flow rate is set toallow the Al_(x)Ga_((1-x))N layer to be grown at a relatively highdoping concentration. Here, the Al_(x)Ga_((1-x))N layer can have ann-type dopant concentration of about 1×10¹⁸ to about 1×10¹⁹/cm³.

In some implementations, the n-type dopant source can be asymmetricallysupplied, as shown in FIG. 4 a.

According to this embodiment, the first n-type semiconductor layer 131can include a high doped Al_(x)Ga_((1-x))N layer and a low doped orundoped Al_(y)Ga_((1-y))N layer, wherein x gradually decreases in adirection away from the growth substrate 110 and y gradually increasesin the direction away from the growth substrate 110.

In the first n-type semiconductor layer 131, the Al_(x)Ga_((1-x))N layer(0<x<1) and the Al_(y)Ga_((1-y))N layer (0<y<1) can have differentgrowth rates due to a difference in growth pressure. With thisstructure, it is possible to block propagation of a dislocation orchange a propagation route of the dislocation, thereby reducingdislocation density of other semiconductor layers grown in subsequentprocesses. Furthermore, since the compositions of the Al_(x)Ga_((1-x))Nlayer (0<x<1) and the Al_(y)Ga_((1-y))N layer (0<y<1) are changed,stress caused by lattice mismatch can be relieved, thereby securingexcellent crystallinity of other semiconductor layers grown insubsequent processes while preventing damage such as cracks and thelike.

Furthermore, since the pressure and flow rates of other element sourcesexcept the n-type dopant source, and temperature are kept constantduring growth of the Al_(x)Ga_((1-x))N layer (0<x<1) and theAl_(y)Ga_((1-z))N layer (0<y<1), it is possible to provide an n-typesemiconductor layer having superior crystallinity to semiconductorlayers in the art without performing an additional process. In addition,since the pressure change is continuously and consecutively performedduring growth of the first n-type semiconductor layer 131, the processis simple and easy to improve reliability of the process, as comparedwith the case where the pressure change is performed in a pulse form.Furthermore, since a waiting time required for the pressure of thegrowth chamber to reach a target pressure can be omitted, it is possibleto reduce a growth time of the n-type semiconductor layer 130, ascompared with the case where the pressure change is performed in a pulseform.

Furthermore, in the first n-type semiconductor layer 131, a highimpurity concentration layer and a low impurity concentration layer arealternately repeated to suppress deterioration in crystallinity of thesemiconductor layers due to impurities.

However, it should be understood that the present disclosure is notlimited to the method of growing the first n-type semiconductor layer131 described above. The pressure change includes the pressureincreasing period and the pressure decreasing period, which can beirregular. In addition, the n-type dopant source can also be supplied ina pulse form, which can also be irregular.

According to the method described above, the first n-type semiconductorlayer 131 includes the Al_(x)Ga_((1-x))N layer (0<x<1) and theAl_(y)Ga_((1-z))N layer (0<y<1), wherein the Al content of each of theAlxGa(1-x)N layer (0<x<1) and the AlyGa(1-z)N layer (0<y<1) iscontinuously changed in the thickness direction of the first n-typesemiconductor layer 131. In some implementations, Al content s of theAlxGa(1-x)N layer (0<x<1) and the AlyGa(1-z)N layer (0<y<1) arecontinuously and spatially changed from one surface of the first n-typesemiconductor layer 131 to the other surface of the first n-typesemiconductor layer 131. Accordingly, the first n-type semiconductorlayer 131 can have band gap energy continuously changing in thethickness direction in some region thereof. For example, FIGS. 23a, 23b,24a , and 24 b show band gap energy of each of first n-typesemiconductor layers 131 in the thickness direction thereof, in whichthe first n-type semiconductor layers 131 were formed under growthconditions of FIGS. 3a, 3b, 4a and 4b . In FIGS. 23a, 23b, 24a and 24b ,each of reference numerals including ‘a’ denote the band gap energy ofan AlGaN layer grown in each time zone in FIGS. 3a, 3b, 4a and 4b . Forexample, in FIG. 3a , the band gap energy of the AlGaN layer grown inthe pressure increasing period 11 corresponds to section 11 a in FIG. 23a.

Next, referring to FIG. 5, an active layer 140 and a p-typesemiconductor layer 150 are formed on the n-type semiconductor layer130.

The active layer 140 can include (Al, Ga, In)N and can emit light havinga peak wavelength in a desired UV region through adjustment of thecomposition ratio of the nitride semiconductor. The active layer 140 canbe formed in a multi-quantum well (MQW) structure wherein barrier layers(not shown) and well layers (not shown) are alternately stacked oneabove another. For example, the active layer 140 can be formed bygrowing a quaternary nitride semiconductor such as AlInGaN to form thebarrier layers and the well layers at a temperature of about 700° C. toabout 1000° C. and a pressure of about 100 Torr to about 400 Torr.

In addition, among the barrier layers, a barrier layer closest to then-type semiconductor layer 130 can have a higher Al content than otherbarrier layers. The structure wherein the barrier layer closest to then-type semiconductor layer 130 is formed to have a higher Al contentthan other barrier layers can effectively prevent overflow of electronsby reducing electron mobility. Further, barrier layers disposed near then-type semiconductor layer can have a greater thickness than some otherbarrier layers, and the uppermost barrier layer can have a greaterthickness or a higher Al content than some other barrier layers. Forexample, the first and second barrier layers can have a greaterthickness than the third barrier layer, and the uppermost barrier layercan have a greater thickness than the third barrier layer.

The p-type semiconductor layer 150 can be grown on the active layer 140and can be formed to a thickness of about 0.2 μm or less at atemperature of about 900° C. to about 1000° C. and a pressure of about100 Torr to about 400 Torr. The p-type semiconductor layer 150 caninclude a nitride semiconductor such as AlGaN, and can be doped tobecome a p-type semiconductor layer by including p-type impurities suchas Mg.

Furthermore, the p-type semiconductor layer 150 can further include adelta doping layer (not shown) to reduce ohmic contact resistance andcan further include an electron blocking layer (not shown).

The electron blocking layer can include an AlGaN layer. In addition, theelectron blocking layer can include a first electron blocking layer (notshown) and a second electron blocking layer (not shown) disposed on thefirst electron blocking layer, wherein the first electron blocking layercan have a higher Al content than the second electron blocking layer.

Next, referring to FIG. 6, a support substrate 160 is formed on thep-type semiconductor layer 150.

The support substrate 160 can be or include an insulating substrate, aconductive substrate, or a circuit substrate. For example, the supportsubstrate 160 can be or include a sapphire substrate, a gallium nitridesubstrate, a glass substrate, a silicon carbide substrate, a siliconsubstrate, a metallic substrate, or a ceramic substrate. In addition,the support substrate 160 can be bonded to the p-type semiconductorlayer 150. Thus, a bonding layer (not shown) can be further formedbetween the support substrate 160 and the p-type semiconductor layer 150to bond them each other.

The bonding layer can include a metallic material, for example, AuSn.The bonding layer containing AuSn can realize eutectic bonding betweenthe support substrate 160 and the p-type semiconductor layer 150. Whenthe support substrate 160 is a conductive substrate, the bonding layerelectrically connects the p-type semiconductor layer 150 to the supportsubstrate 160.

Furthermore, a metal layer (not shown) can be formed between the supportsubstrate 160 and the p-type semiconductor layer 150.

The metal layer can include a reflective metal layer (not shown) and abarrier metal layer (not shown) which can be formed to cover thereflective metal layer.

The reflective metal layer can be formed through deposition andlift-off. The reflective metal layer serves to reflect light and can actas an electrode electrically connected to the p-type semiconductor layer150. Thus, the reflective metal layer preferably includes a materialthat has high reflectivity with respect to UV light and is capable offorming ohmic contact. The reflective metal layer can include, forexample, at least one of Ni, Pt, Pd, Rh, W, Ti, Al, Ag or Au.

The barrier metal layer prevents interdiffusion between the reflectivemetal layer and other materials. Accordingly, it is possible to preventincrease in contact resistance and reduction in reflectivity due todamage of the reflective metal layer. The barrier metal layer caninclude Ni, Cr, or Ti, and can be formed in multiple layers.

Referring to FIG. 7, the growth substrate 110 is separated from thesemiconductor layers. In some implementations, the growth substrate 110can be separated from the GaN layer 123. The growth substrate 110 can beseparated by various methods such as laser lift-off, chemical lift-off,stress lift-off, or thermal lift-off, and the like.

For example, when the growth substrate 110 is or includes a sapphiresubstrate, the growth substrate can be separated from the semiconductorlayers by laser lift-off. According to this embodiment, since the GaNlayer 123 can be formed between the n-type semiconductor layer 130 andthe growth substrate 110, the growth substrate 110 can be easilyseparated from the semiconductor layer using a KrF excimer laser.Accordingly, it is possible to resolve difficulty in separation of thegrowth substrate using laser lift-off in fabrication of a UV lightemitting diode in the related art.

Here, it should be understood that the present disclosure is not limitedthereto and other implementations are also possible. For example,additional layers, for example, a sacrificial layer, can be formedbetween the growth substrate 110 and the semiconductor layers, and thegrowth substrate 110 can be separated from the semiconductor layers bychemical lift-off or stress lift-off.

Referring to FIG. 8, after separation of the growth substrate 110, othersemiconductor layers (for example, residues of the GaN layer 123 and/orthe buffer layer 121) remaining on the n-type semiconductor layer 130are removed to expose one surface of the n-type semiconductor layer 130.In order to expose one surface of the n-type semiconductor layer 130,the residues on an upper surface of the n-type semiconductor layer 130can be removed by a chemical and/or physical process, or etching, andthe like.

Further, in order to increase roughness of the exposed surface of then-type semiconductor layer 130, the method can further include formingroughness (not shown) on the surface of the n-type semiconductor layer130. The roughness can be formed by wet etching and the like. Forexample, the roughness can be formed by photo-enhanced chemical (PEC)etching, or etching using a sulfuric/phosphoric acid solution, and thelike. The roughness can be determined in various ways depending uponetching conditions and can have an average height of, for example, 0.5μm or less. The roughness can enhance light extraction efficiency of theUV light emitting diode according to the present disclosure.

Referring to FIG. 9, device isolation trenches 210 can be formed bypatterning the n-type semiconductor layer 130, the active layer 140 andthe p-type semiconductor layer 150. By forming the device isolationtrenches 210, an upper surface of the support substrate 160 can bepartially exposed. In addition, an N-electrode 170 can be formed on eachof device areas isolated from each other by the device isolation trench210.

The N-electrode 170 can be formed in a region disposed below the surfaceof the n-type AlGaN layer in order to improve ohmic characteristics ofthe AlGaN layer having a high Al content. For example, the N-electrode170 can be formed on a groove formed on the upper surface of the n-typesemiconductor layer 130. With this structure, a contact area between theN-electrode 170 and the n-type semiconductor layer 130 is increased, toimprove current injection characteristics.

Patterning of the n-type semiconductor layer 130, the active layer 140and the p-type semiconductor layer 150 can be performed byphotolithography and etching, and the device isolation trenches 210 canbe formed to have inclined side surfaces.

The N-electrode 170 can serve to supply external power to the n-typesemiconductor layer 130 and can be formed by deposition and lift-off.

Thereafter, the support substrate 160 disposed under each of the deviceisolation trenches 210 is divided along line S1, to provide a UV lightemitting diode 100 a as shown in FIG. 10.

FIG. 11 and FIG. 12 are sectional views illustrating an exemplary methodof fabricating a light emitting diode according to some embodiments ofthe disclosure and a light emitting diode fabricated thereby. The lightemitting diode fabricated by the method according to the embodimentshown in FIG. 11 and FIG. 12 is generally similar to that of theembodiment shown in FIG. 1 to FIG. 10, and further includes an AlN layer125 between the n-type semiconductor layer 130 and the GaN layer 123. Inthe following description, different features of this embodiment will bemainly described.

Referring to FIG. 11, the AlN layer 125 can be formed on the GaN layer123 before formation of the n-type semiconductor layer 130.

Formation of the AlN layer 125 can include alternately growing an AlNlayer at a third pressure and an AlN layer at a fourth pressure at atemperature of about 900° C. to about 1100° C. to form a stackstructure. Each of the Al layers alternately stacked one above anothercan have a thickness of about 5 nm, whereby the stack structure can havea superlattice layer structure. Here, the third pressure can bedifferent from the fourth pressure, and can be lower than the fourthpressure. For example, the third pressure can be between 0 and 100 Torr,and the fourth pressure can be between 0 and 400 Torr.

The AlN layer 125 grown at the third pressure and the AlN layer grown atthe fourth pressure can have different growth rates due to a pressuredifference. With this structure, it is possible to block propagation ofdislocation or to change a propagation route thereof, thereby reducingdislocation density of other semiconductor layers grown in subsequentprocesses.

In the AlN layer 125, an Al mole fraction varies depending upon thepressure at which the AlN layer 125 is grown. When the growth pressureof the growth chamber increases during growth of the AlN layer, there isa high possibility that Al sites become vacancies in an AlN crystal orare redisposed with other impurities. Thus, the AlN layer grown at thethird pressure has a lower possibility of vacancy formation orreplacement of the Al sites with impurities than the AlN layer grown atthe fourth pressure. Accordingly, the AlN layer grown at the thirdpressure can have a higher Al mole fraction than the AlN layer grown atthe fourth pressure.

Next, referring to FIG. 12, the n-type semiconductor layer 130, theactive layer 140 and the p-type semiconductor layer 150 are formed onthe AlN layer 125.

Then, subsequent processes substantially similar to those described withreference to FIG. 6 to FIG. 10 are performed, thereby providing a lightemitting diode 100 a as shown in FIG. 10. However, in this embodiment,the AlN layer 125 remaining on the n-type semiconductor layer 130 afterseparation of the growth substrate 110 can be removed by variousmethods.

FIG. 13 and FIG. 14 are sectional views illustrating an exemplary methodof fabricating a light emitting diode according to some embodiments ofthe disclosure and a light emitting diode fabricated thereby.

The light emitting diode fabricated by the method according to theembodiment shown in FIG. 13 and FIG. 14 is generally similar to that ofthe embodiment shown in FIG. 11 and FIG. 12, and further includes anundoped nitride layer 127 on the AlN layer 125. In the followingdescription, different features of this embodiment will be mainlydescribed.

The undoped nitride layer 127 can be formed on the AlN layer 125 beforeformation of the n-type semiconductor layer 130. Formation of theundoped nitride layer 127 can include alternately stacking anAl_(w)Ga_((1-w))N layer (0<w<1) grown at a fifth pressure and anAl_(z)Ga_((1-z))N layer (0<z<1) grown at a sixth pressure to form astack structure. The stack structure can have a superlattice layerstructure. The Al_(w)Ga_((1-w))N layers (0<w<1) and theAl_(z)Ga_((1-z))N layers (0<z<1) alternately stacked one above anothercan be formed to thicknesses of about 5 nm and about 10 nm,respectively, at a temperature of about 900° C. to 1100° C. Accordingly,a superlattice layer structure including the Al_(w)Ga_((1-w))N layers(0<w<1) and the Al_(z)Ga_((1-z))N layers (0<z<1) can be formed.

Here, the fifth pressure can be different from the sixth pressure, andcan be lower than the sixth pressure. For example, the fifth pressurecan be greater than 0 to 100 Torr, and the sixth pressure can be greaterthan 0 to 300 Torr. With the different growth pressures, theAl_(w)Ga_((1-w))N layer (0<w<1) and the Al_(z)Ga_((1-z))N layer (0<z<1)can be formed to have different composition ratios or the samecomposition ratio. For example, under the same growth conditionsexcluding the pressure, the AlGaN layer grown at a lower pressure canhave a higher Al content than the AlGaN layer grown at a higherpressure.

Alternatively, the pressure change during growth of the undoped nitridelayer 127 can be similar to the pressure change during growth of thefirst n-type semiconductor layer 131. That is, the pressure changeduring growth of the undoped nitride layer 127 can include a cycle of apressure increasing period and a pressure decreasing period, in whichthe Al content of the undoped nitride layer 127 can be similarly changedto the Al content of the first n-type semiconductor layer 131.

The Al_(w)Ga_((1-w))N layer (0<w<1) and the Al_(z)Ga_((1-z))N layer(0<z<1) can have different growth rates due to a difference in growthpressure. With this structure, it is possible to block propagation ofdislocation or to change a propagation route thereof, thereby reducingdislocation density of other semiconductor layers grown in subsequentprocesses. Furthermore, when the Al_(w)Ga_((1-w))N layer (0<w<1) and theAl_(z)Ga_((1-z))N layer (0<y<1) are formed to have different compositionratios, stress caused by lattice mismatch can be relieved, therebysecuring excellent crystallinity of the other semiconductor layers grownin subsequent processes while preventing damage such as cracks and thelike.

Next, referring to FIG. 14, the n-type semiconductor layer 130, theactive layer 140 and the p-type semiconductor layer 150 are formed onthe undoped nitride layer 127.

Then, subsequent processes substantially similar to those described withreference to FIG. 6 to FIG. 10 are performed, thereby providing a lightemitting diode 100 a as shown in FIG. 10. However, in this embodiment,the AlN layer 125 and the undoped nitride layer 127 remaining on then-type semiconductor layer 130 after separation of the growth substrate110 can be removed by various methods.

In this embodiment, the AlN layer 125 can be omitted. In this case, theundoped nitride layer 127 can be grown on the GaN layer 123.

FIG. 15 to FIG. 22 are sectional views illustrating an exemplary methodof fabricating a light emitting diode according to some embodiments ofthe disclosure and a light emitting diode fabricated thereby.

The fabrication method according to the embodiment shown in FIG. 15 toFIG. 22 is generally similar to that of the embodiment shown in FIG. 11and FIG. 12, and is different from the above embodiment in that growingthe n-type semiconductor layer 130 includes forming a second n-typesemiconductor layer 133 and/or a third n-type nitride layer 135. In thefollowing description, different features of this embodiment will bemainly described and descriptions of the same feature will be omitted.

Referring to FIG. 15, a template as shown in FIG. 13 is prepared. Forexample, a buffer layer 121, a GaN layer 123, an AlN layer 125 and anundoped nitride layer 127 are grown on a growth substrate 110. Here, asdescribed above, the buffer layer 121, the GaN layer 123, the AlN layer125 and the undoped nitride layer 127 can also be omitted.

Then, referring to FIG. 16, a second n-type semiconductor layer 133 canbe formed on the undoped nitride layer 127.

The second n-type semiconductor layer 133 can be grown on the undopednitride layer 127 within the growth chamber. In addition, the secondn-type semiconductor layer 133 can include an Al_(u)Ga_((1-u))N layer(0<u<1) and an Al_(v)Ga_((1-v))N layer (0<v<1), and can be doped tobecome an n-type semiconductor layer with n-type impurities such as Si.Here, the Al_(u)Ga_((1-u))N layer (0<u<1) and the Al_(v)Ga_((1-v))Nlayer (0<v<1) can include different concentrations of dopants.

Growth of the second n-type semiconductor layer 133 includes introducingan Al source, a GaN source and an N source into the growth chamber, inwhich growth temperature can be set in the range of about 900° C. toabout 1100° C. and at a growth pressure between 0 and 300 Torr withinthe growth chamber. During growth of the second n-type semiconductorlayer 133, a flow rate of each of the Al source, the GaN source and theN source can be kept constant and the growth temperature can also begenerally kept constant within allowable tolerance.

Furthermore, growth of the second n-type semiconductor layer 133 caninclude supplying the n-type dopant source into the growth chamber inthe form of pulses. As shown in FIG. 3, the n-type dopant source can besupplied in the form of pulses, for example in the form of multiplepulses. Accordingly, during growth of the Al_(u)Ga_((1-u))N layer(0<u<1), when the n-type dopant source is supplied at a second flowrate, the Al_(u)Ga_((1-u))N layer (0<u<1) can be doped in a relativelyhigh concentration. When the n-type dopant source is supplied at a firstflow rate, the Al_(u)Ga_((1-u))N layer (0<u<1) can be doped in arelatively low concentration or can become an undoped layer. Forexample, the Al_(u)Ga_((1-u))N layer (0<u<1) can have an n-type dopantconcentration of about 1×10¹⁸ to about 1×10¹⁹/cm³.

In some implementations, the n-type dopant source can be supplied intothe growth chamber in an asymmetrical way as shown in FIG. 4a , or in anirregular way.

In the second n-type semiconductor layer 133, a high impurityconcentration layer and a low impurity concentration layer arealternately repeated, thereby suppressing deterioration in crystallinityof the semiconductor layers due to impurities. As a result, it ispossible to improve crystallinity of other semiconductor layers grown bysubsequent processes.

Then, referring to FIG. 18, a first n-type semiconductor layer 131 isgrown on the second n-type semiconductor layer 133 and a third n-typenitride layer 135 is grown on the first n-type semiconductor layer 131,thereby forming an n-type semiconductor layer 130.

Growth of the first n-type semiconductor layer 131 according to thisembodiment is generally similar to growth of the first n-typesemiconductor layer 131 according to the embodiment described withreference to FIG. 1 to FIG. 10, and growth of the third n-type nitridelayer 135 is generally similar to growth of the second n-typesemiconductor layer 133.

In this embodiment, one of the second n-type semiconductor layer 133 andthe third n-type nitride layer 135 can be omitted. The second n-typesemiconductor layer 133 and/or the third n-type nitride layer 135 areformed before and/or after growth of the first n-type semiconductorlayer 131, thereby improving crystallinity of the semiconductor layersin the light emitting diode. In addition, the second n-typesemiconductor layer 133 and the third n-type nitride layer 135 acting asstress relief layers include n-type dopants, thereby preventingdeterioration in electron injection efficiency by the stress relieflayers.

Then, referring to FIG. 19, an active layer 140 and a p-typesemiconductor layer 150 are formed on the n-type semiconductor layer130. Next, referring to FIG. 20, a support substrate 160 can be formedon the p-type semiconductor layer 150.

The processes shown in FIG. 19 and FIG. 20 are generally similar to theprocesses described with reference to FIG. 5 and FIG. 6, and detaileddescriptions thereof will be omitted.

Referring to FIG. 21, device isolation trenches 210 can be formed bypatterning the n-type semiconductor layer 130, the active layer 140 andthe p-type semiconductor layer 150. By forming the device isolationtrenches 210, an upper surface of the support substrate 160 can bepartially exposed. In addition, an N-electrode 170 can be formed on eachof device areas isolated from each other by the device isolation trench210.

Patterning of the n-type semiconductor layer 130, the active layer 140and the p-type semiconductor layer 150 can be performed byphotolithography and etching, and the device isolation trenches 210 canbe formed to have inclined side surfaces.

The N-electrode 170 can serve to supply external power to the n-typesemiconductor layer 130 and can be formed by deposition and lift-offtechnology.

Thereafter, the support substrate 160 disposed under each of the deviceisolation trenches 210 is divided along line S1, thereby providing a UVlight emitting diode 100 b as shown in FIG. 22.

The light emitting diodes according to the aforementioned embodiments ofthe disclosure have superior crystallinity to typical light emittingdiodes in the related art, and the growth substrate are removed from thesemiconductor layers of the light emitting diodes, thereby securingexcellent heat dissipation efficiency and luminous efficacy.

Although the above descriptions of the embodiments relate to a verticaltype light emitting diode, from which the growth substrate 110 isremoved, it should be understood that the present disclosure is notlimited thereto and other implementations are also possible. Theaforementioned fabrication methods can also be applied to a flip-chiptype light emitting diode.

It should be understood that the present disclosure is not limited tothe embodiments and features described above, and various modificationsand changes can be made without departing from the spirit and scope ofthe present disclosure, as set forth in the following claims.

What is claimed is:
 1. A UV light emitting diode comprising: a supportsubstrate; a p-type semiconductor layer disposed over the supportsubstrate; an active layer disposed over the p-type semiconductor layer;and an n-type semiconductor layer disposed over the active layer,wherein the n-type semiconductor layer comprises a first n-typesemiconductor layer having a portion with band gap energy continuouslychanging along a thickness direction of the n-type semiconductor layer.2. The light emitting diode of claim 1, wherein the first n-typesemiconductor layer comprises a stack structure formed by alternatelystacking an Al_(x)Ga_((1-x))N layer (0<x<1) and an Al_(y)Ga_((1-y))Nlayer (0<y<1).
 3. The light emitting diode of claim 2, wherein a valueof x gradually increases in a direction away from the support substrate,and a value of y gradually decreases in the direction away from thesupport substrate.
 4. The light emitting diode of claim 3, wherein theAl_(x)Ga_((1-x))N layer and the Al_(y)Ga_((1-y))N layer exhibit a linearchange of the Al content.
 5. The light emitting diode of claim 2,wherein the Al_(x)Ga_((1-x))N layer and the Al_(y)Ga_((1-y))N layer havedifferent n-type doping concentrations.
 6. The light emitting diode ofclaim 2, wherein the Al_(x)Ga_((1-x))N layer and Al_(y)Ga_((1-y))N layerhave different thicknesses.
 7. The light emitting diode of claim 1,wherein the n-type semiconductor layer further comprises a second n-typesemiconductor layer disposed over an upper surface of the first n-typesemiconductor layer.
 8. The light emitting diode of claim 7, wherein thesecond n-type semiconductor layer has a stack structure including anAl_(u)Ga_((1-u))N layer (0<u<1) n-type-doped to have a first impurityconcentration and an Al_(v)Ga_((1-v))N layer (0<v<1) n-type-doped tohave a second impurity concentration.
 9. The light emitting diode ofclaim 8, wherein a value of u gradually increases in a direction awayfrom the support substrate, and a value of v gradually decreases in thedirection away from the support substrate.
 10. The light emitting diodeof claim 8, wherein the first impurity concentration is higher than thesecond impurity concentration.
 11. The light emitting diode of claim 7,wherein the n-type semiconductor layer further comprises a third n-typesemiconductor layer disposed over a lower surface of the first n-typesemiconductor layer.
 12. The light emitting diode of claim 11, whereinthe third n-type semiconductor layer has a stack structure including anAl_(k)Ga_((1-k))N layer (0<k<1) n-type-doped to have a third impurityconcentration and an Al_(z)Ga_((1-z))N layer (0<z<1) n-type-doped tohave a fourth impurity concentration.
 13. The light emitting diode ofclaim 12, wherein a value of k gradually increases in a direction awayfrom the support substrate, and a value of z gradually decreases in thedirection away from the support substrate.
 14. The light emitting diodeof claim 12, wherein the third impurity concentration is higher than thefourth impurity concentration.
 15. The light emitting diode of claim 1,wherein the first n-type semiconductor layer comprises another portionwith constant band gap energy in a thickness direction of the n-typesemiconductor layer.
 16. The light emitting diode of claim 1, whereinthe first n-type semiconductor layer includes a high impurityconcentration layer and a low impurity concentration layer.
 17. Thelight emitting diode of claim 1, wherein the active layer includesbarrier layers and well layers, wherein a first barrier layer is closestto the n-type semiconductor layer as compared to other barrier layersand well layers, and wherein the barrier layers and the well layers arealternately stacked one above the other.
 18. The light emitting diode ofclaim 17, wherein the first barrier layer has a higher Al content thanother barrier layers.
 19. The light emitting diode of claim 17, whereina thickness of the first barrier layer is greater thickness of anotherbarrier layer adjacent to the first barrier layer.
 20. The lightemitting diode of claim 17, wherein the p-type semiconductor layerincludes an electron blocking layer.